1. Field of the Disclosure
The present invention relates to RF filters for use in mobile phones and the like.
2. Description of the Related Art
Mobile phones are getting smarter. In the transition from so called 3rd generation smart phones to 4th and 5th generation smart phones there has been an explosive growth in radio frequencies and bands. To be able to operate correctly, it is necessary to filter out signals from nearby bands.
RF and microwave applications benefit greatly from the use of tunable devices and circuits. With components that can be tuned over a broad range, filters can be made to tune over multiple frequency bands of operation, impedance matching networks can be adjusted for amplifier power level or antenna impedance.
To meet the demand of ever more sophisticated smart phones and RF devices in automobiles and the like, it is necessary to use different frequency bands for different communication channels, and for different RF frequency devices such as smart phones to co-exist in the presence of frequencies that would otherwise interfere with normal operation. One way to do this is to use FBAR technology as filters.
FBAR (Film Bulk Acoustic Resonator) filters are a form of bulk acoustic wave filter that have superior performance with steeper rejection curves compared to surface acoustic wave filters. They have low signal loss and consequently enable longer battery life and more talk time in mobile telecommunication technology.
When most applications were third generation (3G), only four or five different bands benefited from using FBAR (Film Bulk Acoustic Resonator) filtering. Now, as worldwide carriers move to 4G (fourth generation), filter specifications are much more stringent.
Barium strontium titanate (BST) is a mixed titanate that exists as a centrosymmetric piezoelectric material having a perovskite structure at room temperature. BST has a high dielectric constant, low dielectric loss and low leakage current density and has been used as the dielectric of capacitors.
BST generally has a high dielectric constant so that large capacitances can be realized in a relatively small area. Furthermore, BST has a permittivity that depends on the applied electric field. Consequently, thin-film BST has the remarkable property that the dielectric constant can be changed appreciably by an applied DC-field, allowing for very simple voltage-variable capacitors whose capacitance can be tuned by changing a bias voltage across the capacitor. In addition, the bias voltage typically can be applied in either direction across a BST capacitor since the film permittivity is generally symmetric about zero bias. That is, BST typically does not exhibit a preferred direction for the electric field. These characteristics enable BST to be used as a dielectric within alternating current circuits, such that at a characteristic voltage that depends on the dimensions, the dielectric material resonates and can thus serve as a filter by absorbing electrical energy and changing it into acoustic energy.
U.S. Pat. No. 7,675,388 B2 to Cardona and York describes a switchable tunable acoustic resonator using BST material. The acoustic resonator comprises a pair of electrodes with a barium strontium titanate (BST) dielectric layer disposed therebetween. The device is switched on as a resonator with a resonant frequency if a DC (direct current) bias voltage is applied across the BST dielectric layer. The acoustic resonator is also switched off if no DC bias voltage is applied across the BST dielectric layer. Furthermore, the resonant frequency of the acoustic resonator can be tuned based on a level of the DC bias voltage, with the resonant frequency increasing as the level of the DC bias voltage increases.
In one design described therein, U.S. Pat. No. 7,675,388 B2 describes such acoustic resonators formed on sapphire substrates. In another design described therein, such acoustic resonators are formed over an air gap disposed between the second electrode and a substrate. Also described are acoustic resonators formed over an acoustic reflector disposed between the second electrode and a substrate, where the acoustic reflector is comprised of a plurality of alternating layers of platinum (Pt) and silicon dioxide (SiO2) which reduces the damping of the resonance of the acoustic resonator caused by the substrate.
The BST based acoustic resonator functions can be switched on or off by applying a DC bias voltage and its resonant frequency can be tuned by varying the DC bias voltage. Thus BST based acoustic resonators have many versatile uses in electronic circuits, such as switchable, tunable filters and duplexers for transmitting and receiving a radio frequency signal over an antenna.